1. Technical Field
The present invention relates in general to control systems for use with oil well pumping units and in particular to methods and systems for variable speed control of an oil well pumping unit. Still more particularly the present invention relates to a control system for varying the speed of an oil well pumping unit in response to variations in load within the oil well pumping unit.
2. Description of the Related Art
The recovery of oil from subterranean reservoirs is a well known and long established art. Very few oil well are self flowing and most wells require pumping to lift oil to the surface. This is generally accomplished utilizing a submersible pump within a borehole which is actuated by a reciprocating string of sucker rods extending downward through the borehole to the pump. These sucker rods are generally attached to a polish rod at the surface which passes through a stuffing box and which is generally attached to a mechanical device which produces the necessary reciprocating movement.
Typically, the polish rod is attached to a so-called "walking beam" which is pivotedly mounted to a post. A counter balance weight is generally attached to the other end of the walking beam and the beam is rocked by the action of an electric or gas powered motor, raising and lowing the sucker rods.
In typical operation a pump is operated for some predetermined period of time and then turned off, to permit additional oil to seep into the borehole. If the pump is continually operated the lowing of the level of oil within in the borehole to a point below the pump, a so-called "pump off" condition, can result in excessive wear or even catastrophic failure of the pumping unit, as the pump is forced downward onto the fluid level.
Modern oil well pumping units are often equipped with "pump off" detection devices which monitor the load on the sucker rods and the position of the walking beam to create a graphic representation known as a dynagraph. Automatic or manual examination of this dynagraph may then be utilized to stop the oil well pumping unit for a period of time to permit the borehole to once again fill with oil avoiding the "pump off" condition.
While the aforementioned systems are generally successful they do not address the problems encountered with variable oil viscosity which may exist in certain wells. So-called "heavy" oil is oil of such high viscosity that the oil must often be heated by artificial means in order to permit production by conventional pump units. In such wells after heating by steam injection or other devices, the oil becomes sufficiently thin that normal recovery may take place. Thereafter, as the oil cools, the ability of the pump and sucker rod string to fall through the oil is impeded due to the increased viscosity of the oil as it cools. As a result a condition known as "rod float" may occur. In this condition the bridle which attaches the polish rod to the beam may descend at a faster rate then the rod string, causing a separation between the bridle and the end of the polish rod. Thereafter, as the bridle moves upward while the separated rod string is still falling downward, a tremendous impact may occur, causing failure of the polish rod, the bridle or the sucker rod string Additionally, even if pump unit failure does not occur production is decreased as a result of the failure of the pump unit to complete a full stroke.
It should therefore be apparent that a need exist for a method and system whereby the speed of an oil well pumping unit may be automatically varied to accommodate variations in oil viscosity.